Subscriber access provided by UniSA Library
Article 2
The Effect of Pore Confinement of LiNH on Ammonia Decomposition Catalysis and the Storage of Hydrogen and Ammonia Peter L. Bramwell, Sarah Lentink, Peter Ngene, and Petra E. de Jongh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10688 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
The Effect of Pore Confinement of LiNH2 on Ammonia Decomposition Catalysis and the Storage of Hydrogen and Ammonia
Peter L. Bramwell, Sarah Lentink, Peter Ngene, Petra E. de Jongh* Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, The Netherlands, 3583CG. *
[email protected] ABSTRACT: LiNH2 is of interest to several aspects of energy storage such as reversible hydrogen storage, battery technology, catalysis and ammonia capture/storage. We investigated the impact of nanoconfinement in carbon scaffolds on the hydrogen and ammonia release properties of LiNH2 and its catalytic activity in NH3 decomposition. Ammonia release from macrocrystalline LiNH2 begins at 350 °C, while confined LiNH2 releases ammonia from below 100 °C under helium flow. This ammonia release consisted of 30.5 wt% of ammonia in the first cycle and was found to be partially reversible. Above 300 oC hydrogen is also released due to an 1 Environment ACS Paragon Plus
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 36
irreversible reaction between LiNH2 and the carbon support to form Li2NCN. Ni-doped LiNH2/C nanocomposites were active in the catalytic decomposition of ammonia into N2 and H2 with 53% conversion at 400 °C and a gas hourly space velocity of 13000 h-1. This is comparable to the performance of a commercial-type Ru-based catalyst where 79% conversion is observed under the same conditions. This work demonstrates that nanoconfinement is effective for improving the functionality of LiNH2. The versatility of this system offers promise in a number of different areas including hydrogen/ammonia storage and ammonia decomposition catalysis.
1.
Introduction
LiNH2 has garnered a great deal of interest in a number of energy storage applications. Reversible hydrogen storage is a prime example as LiNH2 can store 6.5 wt% of hydrogen which is released through its decomposition to Li2NH (Scheme 1).1–9 However, there are several drawbacks of this system for hydrogen storage, the first being the high temperature required to release hydrogen at a reasonable rate, approximately 300 °C.
Scheme 1. The possible equilibria of lithium amide.2 (∆H = 84 kJ mol-1 NH3)
(1)
2LiNH ↔ Li NH + NH
(2)
LiH + NH → LiNH + H
(∆H = -39 kJ mol-1 H2)
(3)
Li NH + LiH ↔ Li N + H
(∆H = 165 kJ mol-1 H2)
The second drawback is the generation of ammonia during decomposition,10–14 which is a poison for fuel cells.8 Ammonia release can be prevented by mixing LiH with LiNH2 so that the
2 Environment ACS Paragon Plus
Page 3 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
LiH rapidly captures the released ammonia to form LiNH2 and H2 gas. The reaction between LiNH2 and LiH has been extensively studied in order to minimize the ammonia release and maximize the hydrogen release. Studies have shown that a 1:1 mixture of LiNH2 and LiH is optimum and the two components must be thoroughly mixed to prevent ammonia release.15 On the other hand, this ammonia release could be exploited in the storage of ammonia gas for indirect storage of hydrogen.16–18 The decomposition of ammonia to yield hydrogen and nitrogen has recently garnered a lot of interest due to the fact that high purity hydrogen can be produced in-situ from ammonia, which already has an extensive transport and storage infrastructure in place.17 A study on the use of LiNH2 in ammonia decomposition indicated that the active phase was in fact Li2NH, which is formed upon decomposition of LiNH2.18 As the decomposition of LiNH2 only occurs at temperatures above 300 °C, catalytic activity is only observed at 350 °C and above. Although this is comparable to the commercially utilized Ru/alumina catalyst and the recently studied NaNH2,19 the temperatures required for full conversion of ammonia are still very high. Progress in this field has also been made through the use of doping Li2NH with transition metal nitrides, where a ternary nitride is formed.20–22 A range of ternary nitrides were tested from Ti to Cu where Mn demonstrated the highest activity at temperatures above 327 °C.21 Reduction of the particle size, or nano-sizing, is an established strategy for improving the kinetics, for instance for the hydrogen release from light metal hydrides.23,24 Interaction between the metal hydride and carbon-based supports give an additional reduction in hydrogen release temperatures.25–28 Nano-sizing has been applied to several systems such as LiBH4,29 MgH230,31 and NaAlH432,33 supported on carbon. However, there are very few examples of the preparation of supported nanoparticles of LiNH2. This may be because the preparation of such materials is far from trivial. Techniques such as melt infiltration are not applicable for LiNH2 as it
3 Environment ACS Paragon Plus
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
decomposes before melting, however there are a few examples of alternative preparation strategies being found.34–36 Nanocomposites can be prepared by solution impregnation but the low solubility of LiNH2 and other components of the system severely limits the use of this method in this case. An alternative is to use precursors that can be subsequently transformed to LiNH2. Impregnation of LiN3 which can be decomposed to form Li3N is an example from the literature.36 The resulting Li3N/C nanocomposite can reversibly absorb 9 wt% of hydrogen to form LiNH2 and decomposes back to Li3N at 300 °C. We have previously reported a method for preparing carbon supported LiH particles through impregnation with butyllithium with a range of sizes, and demonstrated that nanoconfinement has a large influence on the hydrogen release profile37. In this report we present a procedure for the preparation of LiNH2/C nanocomposites. We build on the solution impregnation using butyllithium to produce LiH particles,38 which can then be treated with gaseous ammonia to yield LiNH2 particles. We show that nanoconfinement in nanoporous carbon has a remarkable effect on the properties of LiNH2 in energy storage applications including reversible ammonia storage, hydrogen storage, and for the catalytic decomposition of ammonia into nitrogen and hydrogen.
2.
Experimental
Materials: All materials were stored in an argon-filled glovebox (Mbraun Labmaster dp, 1 ppm H2O,
